1. Field of the Invention
The invention relates to an integrated altimeter and Doppler velocity sensor arrangement for use in an airborne vehicle. More specifically, the invention relates to such an arrangement which uses one waveform for the altimeter function and a different waveform for the Doppler velocity sensor function.
2. Description of Prior Art
Integrated ground speed and altimetery measuring instruments are known in the prior art as illustrated in, for example, U.S. Pat. No. 3,239,837, Fried, Mar. 8, 1966, U.S. Pat. No. 3,611,370, Frasure et al, Oct. 5, 1971, U.S. Pat. No. 3,860,925, Darboven, Jr., Jan. 14, 1975, U.S. Pat. No. 4,806,935, Fosket et al, Feb. 21, 1989, and U.S. Pat. No. 4,594,676, Breiholz et al, June 10, 1986.
The '837 patent teaches an integrated ground speed and altimetry measuring instrument. However, the transmitted wave in the '837 patent is modulated by a single modulator. Separate channels are used in the receiver for providing ground speed and altimetry outputs.
The '370 patent also teaches an integrated Doppler and altimeter radar. In the '370 patent, the transmitter is arranged for operation in either the ICW (interrupted continuous wave) mode or FM-CW mode. The radar must operate.
In the '925 patent, the system is sine-FM modulated. However, in the '925 patent, altimetry data is extracted from the frequency-modulated Doppler system. It does not employ a separate modulating wave whose data is "corrected" by the Doppler data.
The '935 patent is an improvement of the system in the '925 patent. The '935 patent uses a closed loop operation of signal processing portions for reflected signals including phase shift information. Once again, it does not teach closer to the present invention than the '925 patent.
The '676 patent is presented as of interest only in that it teaches an FM-CW radar altimeter whose output is used to calculate ground speed.
Other radar techniques exist for range and velocity determination, and it is well known that a waveform can be chosen to estimate either range or velocity with high accuracy. It is also well known that it is not possible to simultaneously perform these measurements well with one waveform while still obtaining high accuracy. It has been proven that such a waveform does not exist. However, modern research looks for rather esoteric waveforms that offer the best possible compromise for a given situation.